Liquid crystal display elements are used widely in watches, electronic calculators, electronic notebooks, word processors, note book type-personal computers, video cameras equipped with a monitor, car-navigation systems, etc. These liquid crystal display elements utilize optical anisotropy (.DELTA.n) and dielectric anisotropy (.DELTA..epsilon.) of liquid crystal materials. Display systems of the liquid crystal display elements include a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a dynamic scattering (DS) mode, a guest-host mode, DAP (Deformation of Aligned Phases) mode and the like, according to electro-optical effects which are applied to the liquid crystal display element. In any system, it is desirable that a liquid crystal compound shows a liquid crystal phase at a temperature ranging as broad as possible, and it is necessary that a liquid crystal compound is stable against water, heat, air or the like. It is also necessary that a liquid crystal compound shows high miscibility with other liquid crystal compounds. Although a number of liquid crystal compounds have been already known, there is no liquid crystal compounds which satisfy all the above-mentioned conditions at present, and in practice several liquid crystal compounds are used in admixture with non-liquid crystalline compounds.
Recently, there is an increasing need for a low-voltage operation in association with a decreased electric power consumption and miniaturization of a display element, and thus there is in need of a liquid crystal compound having low threshold voltage (Vth). In order to decrease the threshold voltage, it is necessary to increase dielectric anisotropy (.DELTA..epsilon.).
Examples of a liquid crystal compound having large dielectric anisotropy include an ester derivative represented by the following general formula 1) disclosed in WO89/08102. ##STR2## wherein R represents R.sub.1, R.sub.1 O or R.sub.1 COO, R.sub.1 represents an alkyl group having 1 to 12 carbon atoms, n and m individually represent 0 or 1, (n+m) also represents 0 or 1, a, b, c and d independently represent 0 or 1, and (a+b+c+d) does not represent 0, excluding a case wherein a and b individually represent 0, m represents 0, and one of c and d represents 0, provided that these symbols are different from those used in the present invention and are applied to merely the general formula 1).
However, there is a need for a liquid crystal material which has improved miscibility with known liquid crystal compounds at a low temperature over the compound represented by the general formula 1).
Furthermore, examples of a liquid crystal compound having large dielectric anisotropy include an ester derivative represented by the following general formula 2) disclosed in J.P. KOKAI No. Hei 4-300861. ##STR3## wherein A.sub.1 and A.sub.2 represent a 1,4-phenylene group or the like, R represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, X represents --O--, --CO--O--, --O--CO-- or a covalent bond, Y represents --O--, --CO--O--, --O--CO--, a covalent bond or the like, m and n represent 0 to 4, and q represents 0 or 1, provided that these symbols are different from those used in the present invention and are applied to merely the general formula 2).
However the compound represented by the general formula 2) has an unsaturated bond at the one terminal of the molecule and thus it does not have enough heat stability. Accordingly, there is a need for a liquid crystal material which has an improved heat stability.